Hollow cathodes are used to emit electrons in a variety of industrial applications. As described in a chapter by Delcroix, et al., in Vol. 35 of Advances in Electronics and Electron Physics (L. Marton, ed.), Academic Press, New York (1974), beginning on page 87, there are both high and low pressure regimes for hollow-cathode operation. In the high-pressure regime, the background pressure (the pressure in the region surrounding the hollow cathode) approaches or exceeds 1 Torr (130 Pascals) and no internal flow of ionizable working gas is required for operation. In the low-pressure regime with a background pressure below 0.1 Torr, an internal flow of ionizable working gas is required for efficient operation. It is for operation in the low-pressure regime below 0.1 Torr, and usually below 0.01 Torr, that the present invention is intended.
An important industrial application of low-pressure hollow cathodes is for electron emission in ion sources. These ion sources are of both gridded and gridless types. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, in which case the only electron emitting requirement would be for a neutralizer cathode.
In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a substantial magnetic field in the discharge region, i.e., a magnetic field with sufficient strength to make the electron-cyclotron radius much smaller than the length of the discharge region to be crossed by the electrons. The closed-drift ion source is one type of gridless ion source and is described by Zhurin, et al., in an article in Plasma Sources Science & Technology, Vol. 8, beginning on page R1, while the end-Hall ion source is another type of gridless ion source and is described in U.S. Pat. No. 4,862,032—Kaufman, et al.
There are different types of low-pressure hollow cathodes. The simplest is a refractory-metal tube, usually of tantalum. This type is described in the review by Delcroix, et al., in the aforesaid chapter in Vol. 35 of Advances in Electronics and Electron Physics. For hollow cathodes of the sizes, electron emissions, and gas flows of most interest herein, the use of this cathode type results in a high heat loss and a lifetime of only a few tens of hours, even when operating with clean inert working gas. With the working-gas contamination levels often encountered in industrial environments, the lifetime could be reduced to only several hours.
The lifetime of this type of cathode can be extended by the use of radiation shields, which reduces the heat loss, which in turn reduces the energy of bombarding ions within the hollow cathode—see U.S. Patent Application Publication 2004/0000853—Kaufman, et al. With the proper design of radiation shields, the lifetime with clean working gas can be extended to several hundred hours or more. With contaminated working gas, however, the lifetime could again be reduced to several hours.
Another type of hollow cathode has been developed for electric thrusters used in space propulsion and is described in a chapter by Kaufman in Vol. 36 of Advances in Electronics and Electron Physics (L. Marton, ed.), beginning on p. 265. The distinguishing feature of this type is an emissive insert that emits electrons at a lower temperature, and hence with a lower heat loss, than does the plain metal-tube of the type described above. The major advantage of this type is the long lifetime that is possible, of the order of 10,000 hours. The major disadvantage is the sensitivity of the supplemental emissive material to contamination. This emissive material requires “conditioning” before initial operation and is sensitive to atmospheric exposure after this conditioning. For example, barium carbonate is often used as the supplemental emissive material, which is heated during conditioning to become an oxide. If this emissive material is exposed to air after conditioning, the barium oxide combines with the water vapor in the air to become a hydroxide, which is much less effective as an emission material. Repeated exposure to air is not a problem in the space electric-propulsion application for which these cathodes were originally designed, but is much more serious in industrial applications. The combination of sensitivity to contamination and high fabrication costs make this type of hollow cathode a poor choice for most industrial applications.
What might be called a compromise of the two types of hollow cathodes has been used in industrial applications. In this type, an emissive insert is used, but this insert consists only of tantalum foil. The lifetime is not as long without a low-work-function emissive material such as barium carbonate, but the tantalum-foil insert is less sensitive to atmospheric exposure than an insert that depends on the addition of an emissive material. It should be mentioned that a purge of working gas is normally used for a hollow cathode after exposure to atmosphere and prior to operation. This purge removes most of the impurities from the atmosphere that are adsorbed on the hollow-cathode surfaces, unless they are chemically combined with hollow-cathode material—such as in the formation of barium hydroxide by the water vapor in the atmosphere. However, even with the reduced sensitivity to atmospheric exposure, this type of cathode is still sensitive to impurities (contamination) in the working gas.
Another example of possible hollow-cathode configurations is U.S. Pat. No. 5,587,093—Aston, which differs from other examples given above mostly by additional complexity. There is described a hollow cathode with both multiple radiation shields surrounding a tube through which the working gas is introduced and an emissive insert that is impregnated with an emissive material. Unlike other emissive inserts described herein, this one is directly heated by an electrical current passing through the insert. There are also intervening support structures between both the gas tube and the inner radiation shield and between the inner and outer radiation shields. The contamination-sensitive emissive material and the complicated structure both make it a poor choice for operation with contaminated working gas.
A hollow cathode for industrial applications should have an operating lifetime of at least several hundred hours and be insensitive to repeated exposures to atmosphere between periods of operation. The effect of frequent exposures to atmosphere can be minimized by keeping a flow of clean inert gas through the cathode during these exposures (purging). Shorter lifetimes than several hundred hours would be a problem because the time between maintenance in many industrial applications would then be limited by the cathode lifetime. While longer lifetimes might be of interest for industrial hollow cathodes, the time between maintenance would probably still be limited by other system components. In other words, the cost of a longer-lifetime hollow cathode, together with any special care and handling required, would have to be balanced against the replacement cost of a new hollow cathode of a simpler type.
The best tolerance to atmospheric exposure has been obtained by fabricating the hollow cathode entirely of refractory materials and avoiding the more reactive materials that are used to impregnate or coat an emissive insert. Atmospheric contamination is limited to the surface of refractory materials and is mostly removed by a purge of clean gas before operation. Tolerance to contamination in the working gas, which is usually argon, is a more serious problem. Contaminated working gas reaches the cathode when it is hot and is more likely to react with and/or be absorbed into refractory metals. This contamination results from the use of dirty gas tubing, leaky tubing connections, unsuitable gas regulators, and improper procedures such as opening a new gas bottle without first pumping down the trapped volume between the gas bottle and the regulator. The contaminants involved are usually some combination of oxygen, nitrogen, water vapor, and hydrocarbons. Compared to the use of a clean working gas, typically >99.999% argon, such contamination can reduce the lifetime by a factor of ten or more. Controlling the purity of the working gas at all industrial locations is simply not practical. The approach taken herein has been to increase the tolerance of a hollow cathode to contamination in the working gas.